Plant fibers fall into three groups: seed fibers (e.g., cotton and kapok), stem fibers (bast fibers, e.g., flax and hemp), and leaf fibers (e.g., sisal and kenaf). Bast fibers occur as bundles of fibers, which extend through the length of the plant stems, located between the outer epidermal “skin” layers and the inner woody core (cortex) of the plant. Therefore, bast fiber straw includes three primary concentric layers: a bark-like skin covering layer, a bast fiber layer, and an inner, woody core. The woody core has various names, which depends on the particular plant type. For example, the flax woody core is referred to as “shive.” Thus, “shive” refers to all woody-core materials contained in bast fiber plants.
The bundles of fibers are embedded in a matrix of pectins, hemi-celluloses, and some lignin. The lignin must be degraded, for example by “retting” (partial rotting) of the straw, for example by enzymes produced by fungi (e.g., during dew-retting), or bacteria (e.g., during water-retting). Decortication involves mechanically bending and breaking the straw to separate the fiber bundles from the shive and skin layers, and then removing the non-fiber materials using a series of conventional mechanical cleaning stages.
A substantial proportion of the pectin-containing material that surrounds the individual bast fibers is pectin, with the remaining portion being primarily various water-soluble constituents. Pectin is a carbohydrate polymer, which includes partially-methylated poly-galacturonic acid with free carboxylic acid groups present as calcium salts. Pectin is generally insoluble in water or acid, but may be broken down, or hydrolyzed, in an alkaline solution, such as an aqueous solution of sodium hydroxide.
Removal of the pectin-containing material, or gum, is necessary in many instances to utilize the fiber for its intended purposes. Various methods for pectin removal include degumming, or removing, the pectin-containing substances from the individual bast fiber. For example, U.S. Pat. No. 2,407,227 discloses a retting process for the treatment of fibrous vegetable or plant material, such as flax, ramie, and hemp. The retting process employs micro-organisms and moisture to dissolve or rot away much of the cellular tissues and pectins surrounding fiber bundles, facilitating separation of the fiber bundles from the shive and other non-fiber portions of the stem. Thus, the waxy, resinous, or gummy binding substances present in the plant structure are removed or broken down by means of fermentation.
Following retting, the stalks are broken, and then a series of chemical and mechanical steps are performed to produce individual or small bundles of cellulose fiber. However, a common problem still occurring in non-wood fiber processes is the occurrence of shives, which are dark, undesirable particles in finished paper products. Shives includes pieces of stems, “straw,” dermal tissue, epidermal tissue, and the like.
Shives are substantially resistant to defiberizing processes, rendering their presence problematic. Even following oxidative bleaching, shives continue to have deleterious effects on the appearance, surface smoothness, ink receptivity, and brightness of a finished paper product. Mechanical removal of shive to the level required for a high value product involves the application of significant mechanical energy, which results in fiber breakage and generation of fines. The fines are a yield loss, increasing the production cost. Further, the broken fibers reduce the overall fiber strength so they either cannot be used in some manufacturing processes and/or result in weak textile or paper products.
Thus, conventional methods of non-wood fiber processing are not sufficiently robust to remove, decolorize, and break up the residual shive present in the fibers. Thus, processed and finished fibers can still include dark particles of shive, which are both aesthetically unattractive and reduce the commercial value of the fiber product. Furthermore, conventional alkalizing scouring and peroxide bleaching processes are too mild to significantly degrade the lignin in shive.
Accordingly, there exists an on-going need for a method to selectively degrade the shives present in the non-wood fibers. Thus, the present invention is directed to meeting this and other needs and solving the problems described above.